1. Field of the Invention
This invention relates to a non-regular reflection type holographic mirror and a method of producing the same as well as a reflection type display apparatus for a vehicle, and more particularly to a non-regular reflection type holographic mirror which is mounted on a surface of a windshield of a vehicle to reflect an image of a display unit, which displays information of the vehicle, to the point of view of a driver of the vehicle so as to be visually discerned by the driver and to a method of producing the non-regular reflection type holographic mirror as well as a reflection type display apparatus for a vehicle which employs a holographic mirror.
2. Description of the Related Art
Reflection type display deices for vehicles are already known wherein display light of a display unit is reflected from a reflection mirror to remotely display a display image as a virtual image forwardly of a vehicle. A typical one of such reflection type display apparatus is a head-up display (HUD) apparatus wherein a reflection mirror is used as a combiner. An exemplary one of such conventional head-up display apparatus is shown in FIG. 11. Referring to FIG. 11, a display unit 1 for displaying information of a vehicle thereon is disposed with a display surface thereof directed upwardly in a dashboard 2 of the vehicle. Light emitted from a display image of the display unit 1 is introduced through an opening formed in the dashboard 2 to and reflected by a combiner constituted from a reflection portion 4 on an inner surface of a windshield 3 of the vehicle so that it is introduced to a point E of view of a driver of the vehicle. Consequently, the driver can visually discern a virtual image X of the display image forwardly of the vehicle.
In the reflection type display apparatus of the construction described above, the location of the display unit 1 is restricted by the inclination angle of the windshield 3, and there is little freedom in designing of the location. Thus, an improved display apparatus is proposed wherein a non-regular reflection type hologram having different incidence and emergence angles is mounted on the windshield to increase the degree of freedom in designing the location of the display unit.
A non-regular reflection type hologram mentioned above is produced in the following manner. Referring to FIG. 12, a laser beam 6 emitted from a laser oscillator 5 is reflected by a mirror 7 and then split into two beams of different directions by a beam splitter 8. One of the two beams is reflected by another mirror 9 and converted by an objective lens 10 and a convex lens 11 into parallel light, which is subsequently irradiated as object light 12 upon a surface of a hologram plate 13a. Meanwhile, the other of the two beams is reflected by a mirror 15 and then converted by an objective lens 16 and a convex lens 17 into parallel light, which is subsequently irradiated as reference light 18 upon the other surface of the hologram plate 13a opposite to the surface upon which the object light 12 is irradiated.
If the hologram plate 13a recorded in this manner is developed and fixed, then such a hologram 13 as shown in FIG. 13 is obtained. In particular, referring to FIG. 13, the hologram 13 has interference fringes which are inclined by an inclination angle .theta. with respect to the plane of a photosensitive agent layer 13c applied to a transparent support member 13b and are spaced from each other by a distance d. In this instance, the inclination angle .theta. and the distance d of the interference fringes can be determined in the following manner.
The wavelength of the object light 12 is represented by .lambda., and the angle at which the object light 12 is introduced to the surface of the photosensitive agent layer 13c of the hologram plate 13a is represented by .theta..sub.S while the refraction angle of the photosensitive agent layer 13b is represented by .theta..sub.S '. Further, the incidence angle of the reference light 18 to the transparent support member 13b is represented by .theta..sub.R, and the refraction angle of the transparent support member 13b is represented by .theta..sub.R '. Since the refraction angles of the photosensitive agent layer 13c and the transparent support member 13b may be considered equal to each other.
the inclination angle .theta.=(.theta..sub.S '.theta..sub.R ')/2, and PA1 the distance d=.lambda./2n sin ((.theta..sub.S '+.theta..sub.R ')/2)
where n is a refraction factor of the photosensitive agent layer 13c.
If the hologram 13 is mounted on the windshield 3 as shown in FIG. 14 so as to be used as a mirror, then since it does not reflect light regularly and has an incidence angle and an emergence angle which are different from each other even if the display unit 1 is located nearer to the driver side, the hologram 13 can introduce reflected light therefrom to the point E of view of the driver so that, from the point E of view, the virtual image X of the display image of the display unit 1 can be visually discerned rearwardly of the hologram 13. In this manner, where a non-regular reflection type mirror is used, if the characteristic of non-regular reflection of the non-regular reflection type mirror is suitably set, then the display unit 1 can be located at a position at which it cannot be located with a mirror of the regular reflection type. Consequently, the degree of freedom in designing of the location of the display unit 1 can be increased remarkably.
However, where a mirror of the non-regular reflection type described above is used, the virtual image X of the display image of the display unit 1 is visually discerned, as viewed from the point E of view of the driver, as a forwardly inclined collapsed image. In simple comparison between non-regular reflection and regular reflection, in the case of non-regular reflection, as shown in FIG. 15, light coming out from a point la of the display unit 1 comes to a point Xa; light coming out from another point 1c comes to another point Xc; and light coming out from a further point 1b comes to a further point Xb, thereby forming the virtual image X. In contrast, according to reflection of an imaginary mirror 11 of the regular reflection type, light coming from the points 1a, 1b and 1c comes to points Xa', Xc' and Xb', respectively, thereby forming another virtual image X'. Here, if the distances of the virtual images X and X' from the respective reflecting points are compared with each other, then the distance to the point Xa is longer by 2.times.La than the distance to the point Xa' while the distance to the point Xc is longer by 2.times.Lb than the distance to the point Xc'. Accordingly, as viewed from the point E of view of the driver, the virtual image of the display image of the display unit 1 is visually discerned as a forwardly inclined collapsed image. Therefore, when compared with the virtual image obtained by ordinary regular reflection, the virtual image obtained by non-regular reflection gives an unfamiliar feeling to the driver and deteriorates the discernibility of the driver.
Another head-up display apparatus wherein a reflection type hologram is employed as a combiner is shown in FIG. 16. Referring to FIG. 16, a display unit 1 for displaying various operation information of a vehicle thereon is disposed with a display surface thereof directed upwardly in a dashboard 2 of the vehicle. Light emitted from a display image of the display unit 1 is introduced through an opening 2a formed in the dashboard 2 to and reflected by a combiner constituted from a reflection type hologram 4, which is mounted fixedly on the dashboard 2 independently of a windshield 3 of the vehicle, so that it is introduced to a point E of view of a driver of the vehicle so as to allow the driver to visually discern the display image as a virtual image VI. In this instance, the hologram serves as a mirror for a particular wavelength band.
Depending upon the manner in which a hologram is recorded, the hologram can be provided with a function as a concave mirror or a convex mirror or as a mirror having a characteristic of the regular reflection type (in the case of .theta..sub.1 =.theta..sub.2 in FIG. 9) or the non-regular reflection type (in the case of .theta..sub.1 .noteq..theta..sub.2 in FIG. 16) for a particular wavelength band similarly as described above. The hologram may be used in various manners such that it is mounted independently of the windshield of a vehicle or it is applied to the surface of the windshield or else it is recorded in a glass laminate of the windshield by applying emulsion of a photosensitive agent upon production of the windshield.
While the recording manner of a hologram combiner includes the regular reflection type and the non-regular reflection type as described above, when hologram combiners of the regular reflection recording type and the non-regular reflection recording type are compared with each other, a hologram combiner of the regular reflection recording type little suffers from the problems of fading out and chromatic aberration, but it has a problem that, since a regular reflection image from the rear surface of the hologram plate is formed on the same line of sight, it cannot be separated and cancelled effectively and makes a noise image. While a surface processing technique such as non-reflective coating of the surface of a hologram plate is available as a countermeasure against the program, this makes a factor of an increase of cost.
Meanwhile, in the case of a hologram combiner of the non-regular reflection recording type, while regular reflection images on the front and rear surfaces of the photographic plate can be separated readily from each other and an unnecessary one of them can be cancelled, fading and chromatic aberration increase to drop the discernibility as the displacement of the diffraction direction by the hologram from the regular reflection direction of the emulsion layer increases. Further, since the inclination of an image is different from that of regular reflection, a display image is inclined and the angle of visibility is reduced (the image is collapsed in a vertical direction).
FIG. 17 shows a display optical system of a head-up display apparatus (HUD) which employs a non-regular reflection type hologram, and FIG. 18 illustrates movement of the position of a display unit 1 when a virtual image VI is observed by way of a half mirror 4' (which corresponds to a regular reflection type hologram) on the same line of sight.
As can be seen from comparison between FIGS. 17 and 18, when the non-regular reflection type hologram (corresponding to the case of .theta..sub.1 &gt;.theta..sub.2 in FIG. 16) is used, the virtual image VI is inclined toward the driver comparing with the regular reflection type hologram or half mirror 4'. The reason is substantially illustrated in FIG. 19.
Referring to FIG. 19, a hologram reflection mirror 4 of the non-regular reflection type has interference fringes 4a which are inclined relative to the emulsion or photosensitive agent layer of the hologram 4 (refer also to FIG. 13). Since normally a hologram is designed so that a central portion of a display image may be reproduced at a central portion of the surface of the hologram, when the driver looks into the hologram 4 as seen in FIG. 19, a central portion of an image is observed at or around the center of the surface of the hologram 4. In this instance, it may be considered imaginarily that such a mirror M.sub.1 as indicated by a broken line in FIG. 19 is present at or around the center of the hologram 4. It is to be noted here that any influence of a refraction effect of a difference in refraction factor between the hologram emulsion and the external atmosphere is ignored. In this manner, it may be supposed that a central portion of the image is formed from diffracted (reflected) light from the mirror M.sub.1 coming to the driver. An upper end R.sub.2 of a display image R is diffracted at a portion of the surface of the hologram 4 above the central portion and may be considered to be reproduced as a virtual image VI.sub.2 by diffraction (reflection) from another mirror M.sub.2. Similarly, a lower end portion R.sub.3 of the image R is diffracted below the center of the hologram 4 and is visually discerned as a virtual image VI.sub.3 by diffraction from a further mirror M.sub.3 by the driver.
Since display light from the display unit 1 is diffracted on the surface of the hologram differently for individual light emitting points in this manner so that the routes of the light coming to the eyes of the driver are different from each other and the imaginary reflection surfaces corresponding to the light emitting points are different from each other, the image is inclined. While the tendency of such inclination of an image is common to non-regular reflection type holograms, the direction of the inclination depends upon the direction of the inclination of the interference fringes with respect to the emulsion layer, and when the direction of the inclination is opposite, as shown in FIG. 20, to that of FIG. 19 with respect to the vertical direction of the emulsion layer, an upper end portion of the virtual image VI is inclined remotely from the driver. It is to be noted that reference character VI' denotes an imaginary reflection image when an imaginary mirror M' is present at the location of the hologram 2.
In the case of .vertline..theta..sub.2 .vertline.&gt;.vertline..theta..sub.1 .vertline. wherein the absolute value .vertline..theta..sub.2 .vertline. of the reflection angle is greater than the absolute value .vertline..theta..sub.1 .vertline. of the incidence angle, when a general situation wherein a hologram is mounted on a vehicle is considered, the situation is equivalent to the case wherein the display unit is installed, in the positional relationship of the display unit and the observer (driver) to the combiner, nearer to the observer than that with a hologram of regular reflection. Such positional setting is adopted frequently in order to avoid a hologram from being located in the proximity of the root of the windshield of a vehicle where some structural elements of the vehicle such as an air duct and a wire harness and the display unit interfere with each other. Therefore, such a hologram combiner of the non-regular reflection type as shown in FIG. 17 is in many cases suitable for an optical system for a head-up display apparatus, and in this instance, means to solve the problem that an upper end portion of a virtual image is inclined toward the driver is required.
As a possible countermeasure against the problem, it may be supposed readily that the image VI can be caused to rise by inclining the display unit 1 in the direction indicated by an arrow mark .theta. in FIG. 19. In this instance, however, the height of the image observed by the driver (the angle of visibility) is reduced as can be seen apparently from FIG. 21 to vary the aspect ratio, resulting in deterioration of the discernibility.
The problem of inclination of a virtual image arises also with a regular reflection type hologram when it constructs a concave mirror.